Proximity sensor

ABSTRACT

The proximity sensor detects persons or objects moving into or out of the field of an antenna energized by a low frequency signal. More particularly, a bridge circuit is coupled between a source of low frequency, an antenna, and a detector. The bridge is balanced to divide the energy from the source between the antenna and the detector. A switch is connected to cause a control function responsive to the detection of unbalances which occur in the bridge when the field of the antenna is upset. By changing the efficiency of the bridge, the depth of the antenna field may be varied without affecting the operating threshold of the detector.

United States Patent [72] Inventor Solly L. Fudaley Palos Park, Ill. [21] Appl. No. 667,600 [22] Filed Sept. 13,1967 [45] Patented Apr. 6, 1971 [73] Assignee R-F Controls, Inc.

Chicago, Ill.

[54] PROXIMITY SENSOR 12 Claims, 10 Drawing Figs.

[52] US. Cl 340/258, 307/116, 317/146, 317/153 [51] Int. Cl G08b 13/26 [50] Field of Search 340/258 (C); 317/146, 153; 307/1 16; 340/258 [56] References Cited UNITED STATES PATENTS 2,709,251 5/1955 Schmidt 340/258X 2,943,306 6/1960 Gray et al. 340/258 3,406,802 10/1968 Needham et al. 340/258X 3,462,692 8/1969 Bartlett 340/258X Primary Examiner-Alvin H. Waring Assistant ExaminerDavid L. Trafton Attorney-J. Warren Whitesel ABSTRACT: The proximity sensor detects persons or objects moving into or out of the field of an antenna energized by a low frequency signal. More particularly, a bridge circuit is coupled between a source of low frequency, an antenna, and a detector. The bridge is balanced to divide the energy from the source between the antenna and the detector. A switch is c0nnected to cause a control function responsive to the detection of unbalances which occur in the bridge when the field of the antenna is upset. By changing the efficiency of the bridge, the depth of the antenna field may be varied without affecting the operating threshold of the detector.

PATENTED m 6197| SHEET 1 or 3 PATENTEDAPR elm:

sum 3 OF 3 PROXIMITY SENSOR This invention relates to proximity sensors and more particularly to sensors having improved sensitivity and reliability.

Proximity sensors of the described type are used to perform a suitable control function when an object enters or leaves an area near the sensor. For example, the sensor may be part of a boundary control for guarding a restricted or dangerous area, a guidewire system for guiding moving objects such as autos and trucks, or a safety control device for stopping an automatic machine tool if the operator gets too close. These are critical uses where human life or well-being are at stake. There are, of course, many other less critical uses which might also be cited to illustrate possible uses of these proximity sensors.

In each of these examples, it is not enough to provide the degree of reliability which is commonly found in other comparable electronic circuits. The proximity sensor must have the highest possible degree of reliability in order to protect the human. Accordingly, it is apparent that a safe system is a goal which should always be sought afteran increment of added safety is well worth any efforts required to produce such an increment.

Primarily, this invention is directed toward the production of proximity sensor systems which are more stable, more sensitive, and more reliable. The dichotomy is that the sensor should be designed to respond to an ever smaller amount of movement, but that each reduction of detectable distance introduces stability problems As the detectable movement becomes smaller, any drift takes the sensor into an area wherein it either gives false detections or fails to give a detection when required to do so. With my design, I am able to reliably detect movements in the order of 1 inch or less under a wide range of ambient environmental conditions and with an extremely high degree of safety.

Accordingly, an object of the invention is to provide new and improved proximity sensors. In greater detail, an object is to provide extremely reliable and highly selective sensors. Another object is to provide sensors having a high level of stability despite widely fluctuating environmental conditions.

A further object is to provide sensors having built-in safety features which'prec'lude an accident immediately following a human failure which resulted in a faulty adjustment. In this connection, an object is to provide a safety feature which forces a person to go away from an area of danger before he is able to turn on a newly adjusted proximity sensor, thereby protecting him from his own ineptness or inability to make a proper adjustment.

In keeping with an aspect of the invention, these and other objects are accomplished by means of an antenna which is fed from a low frequency source of signals. The antenna establishes a low frequency field around the boundary of an area which is protected by the sensor. An isolated section is provided to completely isolate the antenna from its driving source so that there cannot be any variation in the applied frequency regardless of how grossly the boundary field might be upset. Also, a bridge circuit is arranged to give a snap-action-like signal to a detection circuit used to give a suitable alarm or otherwise cause a control function when the boundary field is upset. This way, an extremely critical balance may be built into the isolated bridge so that the detector has a hair trigger response to the slightest stimulus from an upset of the boundary field.

The nature of a preferred embodiment of the invention used to accomplish these and other objects should become more apparent from a description of the accompanying drawings, in which:

FIG. 1 is a block diagram showing the functional divisions of my proximity sensor circuit;

FIGS. 27 show exemplary uses of the invention in which the sensor guards a land area (FIG. 2), sorts and counts (FIG. 3), guides an auto (FIG. 4), protects an automatic machine tool and operator (FIG. 5), counts parts being ejected from a tool and die combination (FIG. 6), and provides a more accurate measurement of an unknown impedance (FIG. 7);

FIG. 8 is a schematic circuit diagram of a circuit incorporating a preferred embodiment of the invention;

FIG. 9 shows a series of voltage curves illustrating the distribution of energy as the depth of field is adjusted; and

FIG. 10 shows a number of fragment circuits taken from the antenna bridge in order to explain the bridge operation.

Functionally, the circuit (FIG. 1) of the proximity sensor includes a driving source of low frequency I0, two transformers II, 12 for isolating a circuit section 13, a balanced antenna bridge circuit 14, a multistage narrow band amplifier 15, a detector 16, a DC amplifier I7, and high and low level response circuits I8, 19, respectively.

The isolation section 13 completely isolates the bridge 14 from its low frequency driving source 10. The low frequency is held very stable irrespective of any balanced or unbalanced conditions in the bridge I4 and other circuits connected thereto. Therefore, the oscillator 10 is very stable, even under the worst conditions which are likely to be encountered during operation.

The bridge 14 has an adjustable element 20 in one arm and an antenna circuit 21 in another arm. The other two arms of the bridge 14 are formed by resistors 22, 23. Primarily, the antenna circuit presents an impedance which results from the distributed capacitance and inductance of both an antenna and a coaxial cable used to connect the bridge 14 to the antenna 21.

Under conditions selected by an operator as a quiescent state, the capacitance 20 is adjusted to partially balance the inductive and capacitive reactance of the antenna 21. Under the adjusted balance condition the potential difference across the points 24, 25 is such that only a small current flows through the primary winding of a transfomier 26 used to couple the bridge I4 to the amplifier 15. Since the small current flows in the transformer primary, there is a small signal out of the amplifier 15 which is detected at 16. An associated meter 27 gives a reading which indicates a quiescent condition. When the operator observes this quiescent reading, he knows the bridge 14 is in the adjusted balance.

The proximity sensor circuit is designed to provide a hair trigger response since the small current through the detector 16 is at a threshold level and not at a zero level under fixed signal conditions. At this threshold level, there is an output from the low level switch 19, but none from the high level switch 18. If the current increases or decreases as a result of changes in the adjusted balance in the bridge, the outputs of these switches change.

In greater detail, the fixed signal current is indicated by a point c on a current representing sloping line shown beneath the detector 16. Wheh the low frequency boundary field is changed in one mannersay an object enters the fieldthe bridge 14 is unbalanced in one direction, and the current drops toward a point a. When the boundary field is changed in another mannersay an object leaves the fieldthe bridge is unbalanced in the other direction, and the current increases toward point b. If control functions are performed when current drops or raises to points a or b, respectively, an objective of the circuit designer is to bring the points a or b close together to reduce the distance d as much as possible. This means that the fixed signal current represented by the point c, must be maintained with great precision and stability. In a sense, it might not be too bad if instability should cause the fixed signal current to drift from point c to below point a or above point b because that would only result in a useless indication of a change in the antenna field. However, it would thereafter be disastrous if an unbalanced bridge condition changed the current to the point c because then an unbalanced condition would go undetected. Of course, that cannot be allowed to happen. This highlights the dichotomy. On one hand, the detection gate represented by the distance d must be made as narrow as possible. However, as the gate 11' becomes narrower, it becomes much more difficult to hold the point c within the operating range because tolerances approach zero.

When the operator has set the bridge to an adjusted balance condition, a needle on meter 27 will be pointing to a zero or midscale reading. The scale may increase in each direction in order to help the operator set the balance. Therefore, may represent the point c, and the l on either side of the 0 may represent the points a, b. Thus, an unbalance is detected if the needle moves beyond 1 and l on either side of 0.

The circuit is arranged so that the equipment controlled thereby will not operate unless the high level switch 18 has no output current, and the low level switch 19 does have output current. Thus, the low level relay 28 is operated, and the high level relay 29 is unoperated during normal operation. However, the circuit arrangement is such that even after the bridge 14 is in its adjusted balance, the relay 28 cannot operate because its circuit is open at contacts 30, 31. Therefore, if the switch 31 is put some distance from the machine, the operator must go there and close it before he can turn on a controlled machine, for example. If he did a good job adjusting the balance of the bridge, relay 28 operates when switch 31 closes, and then it locks over its own holding contacts 30. This way, it is not possible for the operator to make a misadjustment of the bridge balance so that the proximity sensor cannot function properly, and then turn on the power while he has his hand under the ram of a punch press controlled by the sensor, for example.

Once the bridge 14 is in the adjusted balance, the low level switch 19 provides a continuous output current to operate the relay 28. As it operates, it closes its contacts to complete a circuit at 32. Relay 29 is unoperated, and circuit 33 is open. This combination of open and closed circuits enables the operation of the controlled equipment. If a foreign object now enters or leaves the low frequency boundary field surrounding the antenna, the adjusted balance of bridge 14 is upset and either the relay 28 releases or the relay 29 operates to cause a suitable control function, such as stopping a machine tool, for exam- While the invention finds many uses, a number are here shown by way of example. Thus, in FIG. 2, the antenna 21a surrounds and protects a geographical area, here generically represented by a house 34.

In FIG. 3, the antenna 21b is placed adjacent a production line to count objects such as 35) coming down a conveyor belt 36. For example, the signals which are generated responsive to disturbances of the field in the antenna 21b may be used to control a device 37 for deflecting the products on the conveyor belt into either of two bins (not shown). FIG. 4 shows the antenna 210 on an auto 38 which is being guided along a highway by means of a roadside guide wire 39. FIG. shows a machine tool having an antenna 21d adjacent a danger area, such as the ram on a punch press, for example. Therefore, if the operator has his hand adjacent antenna 21d during a dangerous step in the cycle of the machine tool, the low frequency field is upset and the machine tool stops.

FIG. 6 shows a tool and die combination 40, 41 having a discharge chute 42 adjacent thereto. As the tool 40 is retracted from the die 41, the punched piece part is discharged through the chute 42. As this part passes through the field of an antenna 22e which is adjacent the chute, the machine (not shown) is enabled to operate again. If the discharge of the piece part is not detected as it leaves the chute 42, the machine is not enabled to operate again. This prevents damage to the expensive tool and die.

FIG. 7 shows a method of using the invention as an instrument for measuring an unknown impedance. In greater detail, an unknown impedance Zu is coupled into the antenna bridge 14a in lieu of the antenna 21. Then, one or a combination of impedances 44 are selectively connected into the adjustable branch of the bridge until an adjusted balance is achieved. The sensitivity and positive switching action provided by the invention helps the operator to balance the circuit with greater accuracy.

These are only an exemplary few of many uses which could be cited to describe the invention. Still other uses will readily occur to those skilled in the art.

The nature of the circuits actually used to construct an exemplary embodiment of the inventive proximity sensor is shown in FIG. 8. Dot-dashed lines divide these circuits into functional units which are the same as those represented by the blocks in the diagram of FIG. 1. A simple comparison of reference numerals establishes the relationship between the blocks and the functional circuits.

The oscillator 10 includes a pair of PNP transistors 50, 51 coupled in emitter follower and common emitter configurations, respectively. The oscillator circuit also includes a bridge 52 in an arrangement of a type which is sometimes called a twin-T filter. This twin-T-type of oscillator circuit is well known and characterized by a very sharp cutoff and narrow band-pass response. An adjustable resistance 53 enables the operator to select a particular frequency of operation. This way, adjacent machine tools may be adjusted to operate at noninterfering frequencies.

The remainder of the oscillator components include a pair of resistors 54, 55 coupled as a voltage divider to provide a base bias for the transistor 50. The resistor 56 is an emitter load for transistor 50. Capacitor 57 provides an AC coupling between the emitter output of transistor 50 and the base of transistor 51. The resistor-capacitor circuit 58 provides both a bias and a degenerative feedback control for the transistor 51. The resistor 59 is a voltage dropping circuit. The capacitor 60 provides a decoupling for the power supply. The connection 61 provides a phase shift feedback for causing the circuit 10 to go into oscillation.

The primary winding of the transformer II acts both as an RF frequency choke coil and a transformer coupling between the oscillator 10 and the isolation section 13. The coupling capacitor 63 prevents the low DC resistance of the secondary winding of the transformer 11 from short-circuiting the bias circuit. 7

As those skilled in the art know, the oscillator circuit 10 goes into oscillation because the signal applied to the base of transistor 50 goes through a phase shift by the time that it reaches the twin-T filter 52. In the filter, it experiences another phase shift of approximately 180. Hence, the signal which is fed back over wire 61 to the base of transistor 50 has undergone a phase shift of about 360 to reinforce the voltage swing at the emitter. This causes the circuit to go into oscillation.

The isolation section 13 is completed by a PNP transistor 64 having a split load configuration. The resistors 66, 67 form a voltage divider for applying a base bias to the transistor 64. The resistor 68 acts as a collector load, and the inductance of the primary winding of transformer 12 acts as an emitter load. Isolation section 13 is designed to efficiently transmit the low frequency signals from the oscillator 10 to the antenna bridge 14. However, nothing which happens in the bridge circuit 14 can be transmitted to or otherwise produce any effect upon the output of the oscillator 10. Hence, the low frequency output signal is maintained very stable.

The low frequency oscillator output signal is applied through the transformer 12 to the antenna bridge circuit 14 where capacitor 20 is adjusted to almost balance the distributed inductance and capacitance of the coaxial cable and the antenna 21. The adjustable potentiometer 71 supplements the resistance 23 and makes it exactly equal to the resistor 22.

If any object now enters or leaves the low frequency boundary field set up by the antenna circuit 21, its impedance is changed and the adjusted balance of the bridge is upset. Before upset, and while the bridge is in the adjusted balance only a small current flows through the primary winding of the transformer 26, and a small voltage is induced in its secondary winding. By definition, that small voltage is a quiescent signal. After the adjusted balance of the bridge 14 is upset, the impedances 20, 21 are no longer in the same relative balance, and the difference in potential across the points 24, 25 changes. Hence, more or less current flows through the primary winding of the transformer 26, and the amplifier 15 receives a change of input voltage which, in turn, causes changes in output voltage.

Means are provided for adjusting the depth of the antenna field in which the unbalance may be detected. For example, in the geographical area of FIG. 2, it might be desirable to give an alarm if anyone moves into a distance of 5 feet from antenna 21a. Next, consider the machine tool of FIG. 5. If the antenna 21d has a field of, say 3 or more feet, it would not be possible for an operator to work on the machine while it is in operation because it would shut down before he could get within arms length. 0n the other hand, depending upon the nature of the machine, it might be desirable to have it shut down only when the operator puts his hands within, say, 1 inch of the antenna 21d. Obviously, therefore, there must be some means for adjusting this depth of field.

In order to explain the problems related to this depth of field adjustment, it is helpful to assign actual values to a number of voltages which appear in the circuit. An effort has been made to choose voltages which might reasonably be expected in some systems. However, the choice has also had to be somewhat general so that it is equally applicable to many different systems. Therefore, no special significance should be attached to these particular values-any suitable values will do.

It is assumed that the oscillator feeds a low frequency signal having 5 volts through the transformer 12 to the bridge 14. An unbalance occurs if there is a capacity change of 5 uuf. at the antenna 21. The energy passed through the transformer 26 causes 1 volt to appear at the base of the first transistor in amplifier 15. After having been amplified at l5, l7, and rectified at 16, this same voltage is applied as an 8-volt signal to the high and low level switches 18, 19. The normally operated low level relay 28 releases if this applied voltage drops to 7.8 volts, and the normally unoperated high level relay 29 operates if this voltage rises'to 8.2 volts.

Under the foregoing'assumptions, the depth of field adjustment cannot be allowed to cause any significant change in the 8 volts because that would set off an alarm. Thus, the usual volume control which raises or lowers the output voltages is not suitable for adjusting the depth of field.

The means for providing the depth of field adjustment, depends upon the distribution of voltages in the bridge [4. In greater detail, the amplifier includes three cascaded stages, comprising the common emitter transistors 75, 76, 77, designed to give the amplifier 15 a very narrow, stable, and sharply defined band-pass characteristic. Therefore, the amplifier 15 serves the functions of providing a gain and a narrow band-pass filter tuned to the frequency of oscillator 10. An advantage is that the signal-to-noise ratio is greatly improved because the noise is either outside the narrow band-pass or it has a voltage which is very small as compared with the voltage of the signal. While the nature of the noise sources is immaterial, it might be well to note that they may either be picked up on the antenna from nearby generators (such as AC appliances) or appear in the power source.

More particularly, the coupling transformer 26 has a relatively high Q AC impedance increase the sharpness of the frequency response. A variable resistor 79 is coupled across the secondary winding of the transformer 26 to lower the Q or AC impedance by a selective amount. When the entire resistance 79 is connected across the secondary winding, the full Q appearsat the other extreme the resistance 79 is low, and the transformer Q or AC impedance is also low.

The effects of this change in Q or AC impedance may become more apparent from a study of FIGS. 9, l0antenna bridge 14 has been redrawn in fragmentary form in FIG. 10 to explain the operation. The letters X and Y are applied to each be the fragmentary circuits in to become a parallel-series to preserve an indication of where the transformer 26 is connected.

The antenna bridge 14 may be viewed (FIG. 10 BI) as a first or input filter comprising a parallel circuit including an inductor I2, a pair of capacitors 2|, in series, and a pair of resisters 22, 23 also in series. If the pairs of capacitors and resistors are replaced by an equivalent, lumped capacitor and resistor (FIG. 10 B2), it is immediately apparent that the input of bridge 14 forms a circuit that may be tuned to the frequency of the oscillator.

The output of the bridge 14 may be viewed as a second tuned circuit, as shown in FIG. 10 Cl. Here, the tuned circuit includes two parallel circuits, (each being a resistancecapacitance series) in parallel with an inductor (the primary of the transformer 26). Again, the components may be lumped (FIG. 10 C2) to become a parallet-series filter circuit combination. Thus, the output circuit may also be tuned. Those who are skilled in the art will readily recognize the nature of the two circuits of FIGS. 10 B2 and 10 C2 and the effects produced in these two circuits when the impedance 20 and resistance 79 are adjusted.

The two filters of FIGS. 10 B1, 10 C] may now be combined, (FIG. 10 D) to show the complete antenna bridge 14 with its input and output transformers. Three sine waves are also drawn on this complete bridge (FIG. 10 D) in order to identify the points where several voltages of interest appear. Thus, the input from the oscillator 10 is shown by a solid line curve, and the voltage in antenna 2] is shown by a dot-dashed line curve, and the output voltage fed into amplifier 15 is shown by a dottcdline curve, This same convention is also used in FIG. 9 where the curves are drawn only to illustrate a point-they are not in scale.

In each part of FIG. 9, the solid line is drawn with the same waveform and the same phase angle. This identity of voltage waveform reflects the stability of the oscillator 10 and indicates that its output does not change regardless of adjustments.

The capacitor 20 is adjusted so that the bridge is in its adjusted balance when the transformer 26 has high Q or AC impedance. Under the above assumptions, this balance occurs when the meter 27 gives a zero reading responsive to the receipt of an 8-volt signal at the collector of the output transistor in the DC amplifier l7. Recall that this 8-volt signal appears when l volt is applied to the base of transistor 75 and that the output from oscillator 10 is 5 volts. Therefore, in FIG. 9 A the solid line oscillator voltage is drawn rather high, to indicate 5 volts and the dotted line output voltage is drawn rather low to indicate l volt. Neglecting losses, the difference between these voltages is a rather high voltage of 4 volts which are applied to the antenna 21. Also, the various impedances cause a phase shift.

In FIG. 9 A, with the resistor 79 adjusted for high Q or AC impedance, most of the energy from oscillator 10 is fed into the antenna. Therefore, the voltage at the base of the transistor 75 is low relative to the antenna voltage. The point is that a high level of energy is fed into the antenna 21, and it has a great depth of field. Therefore, an alarm is given if anyone approaches the antenna within a relatively great distance, say distances measured in feet.

Next assume that the operator wishes to adjust the depth of the antenna field to be much smaller, say distances measured in inches. The resistor 79 is adjusted so that the secondary winding of the transformer 26 is shunted by a very low resistance, thus lowering the Q or AC impedance. As shown by the dotted line curve in FIG. 9 B, the voltage reaching the base of transistor 75 drops substantially. This would, of course, cause the low level relay 28 to release; therefore, it will be necessary to readjust the bridge in order to restore the adjusted balance and reoperate the relay 28. Therefore, the impedance 20 is readjusted until the meter 27 returns to the midscale reading which represents the receipt of 8 volts at the inputs of the high and low switches l8, 19. As shown by the dotted line curve in FIG. 9 C, the output voltage rises substantially, thereby drawing a greater proportional part of the input energy to reduce the energy fed into the antenna. Thus, say 3.5 volts at transformer 26 may deliver 1 volt to the base of transistor 75. Hence, adjustments of the potentiometer 79 and impedance 20, makes a larger or smaller depth of field and a constant 8-volt control signal appears at the inputs of the switches I8, 19.

The remaining components in the first stage of the amplifier comprises a noise bypass decoupling capacitor 81 which also cooperates with resistor 82 to provide a negative feedback. The RC network 83 provides another negative feedback. Resistor 84 provides emitter bias, resistor 85 is a collector load, and capacitor 86 is a coupling between the stages.

The remaining two amplifier stages (which include the transistors 76, 77) are essentially the same as the described amplifier stage including the transistor 75, except for the capacitors. The noise bypass and degenerative feedback, pro vided via the capacitorfil and the base-to-collector capacitors (such as 83) in each of the three stages of amplifier 15, cause the narrow band-pass characteristics which help suppress the noise and improve the signal-to-noise ratio. The capacitor 87 provides a frequency compensation. A coupling capacitor 89 connects the amplifier 15 to the detector 16.

The detector 16 includes a load resistor 90, a rectifier 91, two smoothing capacitors 92, and a resistor 93 which reduces ripple. A voltage divider 94, 95 applies a base bias potential to PNP transistor 96. The input signal to the detector 16 is rectified at 91 and smoothed at 92, 93 to provide a DC potential which is added to the bias at the base of the transistor 96. When the antenna bridge 14 is in adjusted balance, a signal of 8 volts and an average current reaches the base of the transistor 96, point c on the curve. The needle points at zero. If the bridge thereafter becomes unbalanced in one direction, less current flows through the diode 91, the voltage at the base of transistor 96 falls to less than 7.8 volts (the point a, or less), and the meter needle swings outside the crosshatched zone. If unbalance is in the other direction, the potential changes so that more current flows through the diode 91, the voltage at the base of transistor 96 goes over 8.2 volts, and current rises to point b or above.

The DC amplifier 17 includes four transistor stages which amplify any changes occurring in the detector circuit 16.

These transistors are, alternately, PNP, NPN, PNP, and NPN with self-compensating connections (for small leakage and temperature related currents, i.e. if one transistor begins to conduct more current, it lowers the bias on the next transistor, thus causing it to conduct less current). There is no comparable effect upon the large signal currents.

in greater detail, the first stage includes the transistor 96, an emitter bias resistor 97, and a load resistor 98. The collector of transistor 96 is coupled to drive current into the base of a common emitter transistor 100 which has a collector load resistor 101 and an emitter bias resistor 102. The collector of transistor 100 is, in turn, connected to the base of a transistor 105, having a collector load 103, and an emitter bias resistor 104. Finally, the transistor 105 drives an NPN transistor 106 used in a common emitter configuration and having an emitter bias resistor 107. The collector of transistor 106 is coupled to drive the meter 27, an RC circuit 108, and the high and low level switches 18, 19 respectively. The RC circuit 108 provides a load resistor and a noise bypass capacitor. Since each of the transistors in DC amplifier 17 stabilizes the small leakage and temperature currents the transistor in the next succeeding one of the four cascaded stages, only the transistor 96 needs to be selected with great care. I prefer to use one encapsulated in a small oven, the heat of which is designed to swamp out any ambient environmental temperature changes.

The two switching circuits 18, 19 are provided to respond to the high and low levels of current b, a, respectively, which result from either of the two possible unbalanced bridge con ditions. These switches are energized via gating diodes 110, 111, respectively. Since these two switching circuits are the same except for the biasing required for the high and low scale response, only the low level switch 19 is described herein in detail.

Two common emitter transistors (a PNP and an NPN) are shown at 112, 113, respectively. The base of the PNP transistor 112 is coupled to the collector of the transistor 106 via the input isolation diode 111. The emitter of the transistor 112 is biased by an adjustable series of resistors 114 which set the low level voltage threshold response switch 19 in the assumed case 7.8 volts at the anode ofdiode 111.

The collector load for the transistor 112 is provided by the resistor 115. A resistor 116 couples the collector of the transistor 112 to the base of the transistor 113. The diode 117 clamps the emitter of the transistor 113 slightly above ground potential, and the resistor 118 cooperates with the resistors 114 to divide a voltage and provide a bias for the emitter of the transistor 113. The collector load for the transistor 113 is provided by the resistor 120.

The circuit values and configurations are such that the transistor 112 provides a high gain and the transistor 113 provides a fast switching response. The principle is that the transistors 112, 113 snap on," and the transistor 113 drives itself into saturation, whenever the voltage at its emitter is less than the voltage at the collector of the transistor .106.

The collector of the transistor 113 is coupled via a resistor 121 to the base of an emitter follower 122. The emitter of the transistor122 is, in turn, coupled to the relay 28 by way of a load limiting resistor 123. The emitter follower configuration is used at 122 since it provides a good isolation between the switching transistors 113 and the output relay 28. When the transistor 112 turns on responsive to an adjusted balance of the bridge 14, current flows through the winding of the relay 28. This relay operates and closes its contacts 124 to prepare an alarm device 125. When the transistor 122 turns off responsive to an unbalanced bridge, the alann 125 goes off. The alarm is here shown as an exemplary lamp. However, an audio sounder device, or a red button machine tool stop signal could also be shown.

Similar contacts 126 in the high level switch 18 are also wired to enable the controlled circuit operation. For example, a punch press may operate if contacts 126 are open and stop if they are closed.

The proximity sensor circuit of FIG. 8 operates this way. Responsive to an unbalanced bridge condition, the current through the diode 91 either increases or decreases, depending upon the direction of unbalance. This causes the DC amplifier 17 either to turn off the low level switch 19 when the current through diode 91 drops or to turn on the high level switch 18 when this current increases.

In greater detail, assume that the bridge 14 is unbalanced so that less current flows through the diode 91, and it reaches the trigger point a. The way diode 91 is poled a current of positive polarity is driven into the base of the transistor 96. As the current decreases, the base of the PNP transistor 96 becomes less positive, and it starts to conduct more current. lts emitter becomes more negative as current increases. This makes the base of the NPN transistor more negative, and it begins to conduct less current. As current drops in resistor 101, the base of the PNP transistor becomes more positive. This decreases the current in the transistor 105, and its collector becomes more negative.

As the base of the NPN transistor 106 moves negative, its emitter-collector current decreases and less current flows from the negative source through the resistor 107 to the diodes 110, 111. Less negative current is driven into the base of the PNP transistor 112, and it begins to turn off. As the current through the resistor decreases, the base of the NPN transistor 113 becomes less positive. Its current drops, and the base of the NPN transistor 122 moves toward the positive volt age applied through resistors 120, 121. The transistor 112 turns off and releases the relay 28. The contacts 124 open to turn off the lamp 125, stop a machine, or perform another control function.

The converse happens when the current in diode 91 raises to the level b, and the high level relay 29 operates.

In a proximity sensor actually built and tested by the RF Controls Corp., no drift or change in proximity detection was observed during many months of experimental operations. The antenna field was well within the audiofrequency range. The depth of the antenna field was continuously adjustable over a distance judged to be in the order of four tenthousandths of an inch to about 6 feet. With this range in depth of field, it is-obvious that the sensor has many uses ranging from an electronic quality control measuring machine to an electronic boundary control for a land area.

From the foregoing disclosure, it is apparent that the attached, claims should be construed broadly, to cover all equivalents reasonably falling within the true scope and spirit of the invention.

l'claimi v I 1. An alarm giving proximity sensor comprising a source of a low frequency signal having a fixed phase angle, a bridge circuit having an antenna in one arm thereof, isolation amplifier means for coupling said source to energize said bridge and establish a quiescent electromagnetic field of predetermined phase angle in the vicinity of said antenna, detector means, feedback tuned amplifier means having a narrow band-pass characteristic coupled between said detector and points on said bridge having a predetermined potential difference when said bridge is energiied during said quiescent field condition, adjustable impedance meansin another arm of said bridge for causing a predetermined adjusted level of current to pass from said source through said bridge and said narrow band-pass amplifier means to said detector during said'quiescent field condition, said bridge means being responsive to changes in said electromagnetic field for causing a shifting of said predetermined phase angle. in-said field without appreciably changing either the low frequency or the phase of said signal from said source, said shift of. said predetermined phase angle in said field causing a change of said level of current transmitted from said source through said bridge and, said feedback tuned amplifier, to said detector. means, switching means operated to an intermediate state responsive to said predetermined level of current to said detector, and means responsive to changes of said level of current for. selectively perfonning a control function depending upon whether said level of current goes up-or down.

2. The sensor of claim I wherein said switching means comprises a pair of relays, one of said relays being biased to be operated and the other of said relays being biased to be unoperated responsive to said predetermined level of current in said bridge, said one relay releasing when said current falls below said predetermined level and said other relay operating when said current rises above said predetermined level, and indicator means for selectively indicating the operation and release of said relays.

3. The sensor of claim 2 and means for stabilizing said switching means against changes in ambient temperature conditions to enable a reduction in the difference between said high and low levels. 1

4. The sensor of claim I wherein said antenna is mounted adjacent the perimeter of a dangerous area, and means responsive to the detection of said change in current level bridge for signalling a danger condition.

5. The sensor of claim 1 and an automatic machine tool, said antenna being mounted adjacent said machine, and means whereby said control function is an emergency stopping of said machine.

6. The sensor of claim 1 wherein said isolation means interposed between said source and said bridge provides for efficiently transmitting the output of said source to said bridge while precluding any' changes in the output of said source responsive to changes in the current flow in said bridge.

7. The sensor'of claim 1 and means responsive to changes in the level of current flow, of said bridge for giving a snap-action like response to actuate an alarm indicator.

8. The sensor of claim 1 wherein said low frequency is in the audiofrequency range.

9. The sensor of claim I wherein said narrow band-pass means comprises an amplifier having a plurality of cascaded I stages, each of said stages including a negative feedback circuit having a noise bypass and degenerative feedback tuned to provide a narrow band-pass filter.

1 The sensor of claim 1 and control means for selectively controlling the depth of said electromagnetic field without changing said predetermined level of current.

11. The sensor of claim 10 wherein said control means comprises a bridge circuit for dividing the energy of said predetermined frequency between said antenna and said detecting means, and means for changing said division of said energy without changing said threshold level.

12. The sensor of claim 11 wherein said control means comprises a transformer coupled between said source and said detector, and means for varying the Q 'of said transformer. 

1. An alarm giving proximity sensor comprising a source of a low frequency signal having a fixed phase angle, a bridge circuit having an antenna in one arm thereof, isolation amplifier means for coupling said source to energize said bridge and establish a quiescent electromagnetic field of predetermined phase angle in the vicinity of said antenna, detector means, feedback tuned amplifier means having a narrow band-pass characteristic coupled between said detector and points on said bridge having a predetermined potential difference when said bridge is energized during said quiescent field condition, adjustable impedance means in another arm of said bridge for causing a predetermined adjusted level of current to pass from said source through said bridge and said narrow band-pass amplifier means to said detector during said quiescent field condition, said bridge means being responsive to changes in said electromagnetic field for causing a shifting of said predetermined phase angle in said field without appreciably changing either the low frequency or the phase of said signal from said source, said shift of said predetermined phase angle in said field causing a change of said level of current transmitted from said source through said bridge and, said feedback tuned amplifier, to said detector means, switching means operated to an intermediate state responsive to said predetermined level of current to said detector, and means responsive to changes of said level of current for selectively performing a control function depending upon whether said level of current goes up or down.
 2. The sensor of claim 1 wherein said switching means comprises a pair of relays, one of said relays being biased to be operated and the other of said relays being biased to be unoperated responsive to said predetermined level of current in said bridge, said one relay releasing when said current falls below said predetermined level and said other relay operating when said current rises above said predetermined level, and indicator means for selectively indicating the operation and release of said relays.
 3. The sensor of claim 2 and means for stabilizing said switching means against changes in ambient temperature conditions to enable a reduction in the difference between said high and low levels.
 4. The sensor of claim 1 wherein said antenna is mounted adjacent the perimeter of a dangerous area, and means responsive to the detection of said change in current level bridge for signalling a danger condition.
 5. The sensor of claim 1 and an automatic machine tool, said antenna being mounted adjacent said machine, and means whereby said control function is an emergency stopping of said machine.
 6. The sensor of claim 1 wherein said isolation means interposed between said source and said bridge provides for efficiently transmitting the output of said source to said bridge while precluding any changes in the output of said source responsive to changes in the current flow in said bridge.
 7. The sensor of claim 1 and means responsive to changes in the level of current flow, of said bridge for giving a snap-action like response to actuate an alarm indicator.
 8. The sensor of claim 1 wherein said low frequency is in the audiofrequency range.
 9. The sensor of claim 1 wherein said narrow band-pass means comprises an amplifier having a plurality of cascaded stages, each of said stages including a negative feedback circuit having a noise bypass and degenerative feedback tuned to provide a narrow band-pass filter.
 10. The sensor of claim 1 and control means for selectively controlling the depth of said electromagnetic field without changing said predetErmined level of current.
 11. The sensor of claim 10 wherein said control means comprises a bridge circuit for dividing the energy of said predetermined frequency between said antenna and said detecting means, and means for changing said division of said energy without changing said threshold level.
 12. The sensor of claim 11 wherein said control means comprises a transformer coupled between said source and said detector, and means for varying the Q of said transformer. 